Field of the Invention
Embodiments of the invention generally relate to an electronic implantable device configured to be used in an MRI scanner, in particular one or more of an implantable heart therapy device, a heart monitoring device, a cardiac pacemaker and a cardiovertor/defibrillator.
Description of the Related Art
Generally, implantable heart therapy and/or heart monitoring devices include for example heart stimulators in the form of cardiac pacemakers or cardioverters/defibrillators. Such heart stimulators are generally connected to electrode lines, which have stimulation electrodes and optionally additional defibrillation electrodes in a chamber of a heart or in the immediate vicinity thereof. Typically, a cardiac pacemaker may deliver an electric stimulation pulse to the muscle tissue of a heart chamber via a stimulation electrode (more specifically one or more stimulation electrode poles) in order to thus cause a stimulated contraction of the heart chamber, provided the stimulation pulse has a sufficient intensity and the heart muscle tissue (myocardium) is not in a refractory phase at that precise moment. A contraction of a heart chamber stimulated as such may be referred to herein as a stimulated event. A stimulation pulse having sufficient intensity to cause a stimulated contraction of a heart chamber may be referred to herein as “above-threshold”. Should a natural contraction of the heart chamber occur, such a contraction may be referred to herein as an autonomous action or as a natural or intrinsic event. A contraction, for example of the right atrium of a heart, may be referred to herein as an atrial event, which for example may be a natural atrial event or, in the case of an atrial cardiac pacemaker, may also be a stimulated atrial event. Generally, natural (intrinsic) and stimulated left-ventricular and right-ventricular events may be distinguished in a similar manner.
Typically, a local excitation of the myocardium spreads, starting from the location of excitation, via stimulus conduction in the myocardium and leads to a depolarization of the muscle cells and therefore to a contraction of the myocardium. Generally, after a short period of time, this causes a repolarization of the muscle cells and therefore a relaxation of the myocardium. Typically, during the phase of depolarization, the heart muscle cells are insensitive to excitation, that is to say are in a refractory state. This time is generally referred to as the refractory period. Typically, the electric potentials accompanying the depolarization and repolarization may be sensed, and the course thereof over time (referred to as an electrocardiogram) may be evaluated.
In an electrocardiogram, generally, action potentials accompanying a contraction of the ventricle and reflecting a depolarization of the heart muscle cells may be identified as a Q-wave, whereas the repolarization of the heart muscle cells accompanying the relaxation of the myocardium is reflected as a T-wave.
In healthy individuals, typically, the respective heart rhythm is determined by the sinus node, which is controlled by the autonomous nervous system. Generally, the sinus node excites the right atrium of a human heart by stimulus conduction and also excites the (right) ventricle of the heart via the atrioventricular (AV) node. Typically, a natural heart rhythm starting from the sinus node may be referred to as the sinus rhythm and leads to natural contractions of the respective heart chamber, which may be detected as natural (intrinsic) events.
Generally, such natural (intrinsic) events are detected by recording the electric potentials of the myocardium of the respective heart chamber with the aid of sensing electrodes, which are part of a corresponding electrode line. Typically, the sensing electrode poles may simultaneously be the stimulation electrode poles and may be used alternately as a stimulation electrode pole and as a sensing electrode pole. Generally, a sensing electrode pole pair, which is formed by two adjacent electrode poles, specifically a point electrode (tip electrode) and a ring electrode, the point electrode also serving as a stimulation electrode pole, is typically provided for the sensing, that is to say the sensing of intrinsic events. Generally, a bipolar recording of an intracardial electrocardiogram (IEGM) is thus provided. Typically, intrinsic events and the stimulation in the ventricle are sensed with the aid of a ventricular electrode line, and the stimulation and the sensing of intrinsic events in the atrium (in the right atrium) are implemented with an atrial electrode line, which electrode lines are connected separately to the respective heart stimulator. In addition, generally, a left-ventricular electrode line may also be provided, which typically protrudes via the coronary sinus and a lateral vein branching off therefrom into the vicinity of the left ventricle, where it may have a small-area stimulation and/or sensing electrode.
Typically, to be able to satisfy the different needs of various patients, implantable heart stimulators may be operated in various operating modes. The various stimulation and sensing modes are generally referred to in a standardized manner using a three-letter code, of which the first letter denotes the location of stimulation (V=ventricle, A=atrium, D=ventricle and atrium), the second letter denotes the location of the sensing (V=ventricle, A=atrium, D=ventricle and atrium), and the third letter denotes the type of operation (I=inhibited, T=triggered, D=both inhibited and triggered). In particular for dual-chamber cardiac pacemakers in DDD mode, generally, a ventricular stimulation may be performed synchronously with an atrial heart rate that is as natural as possible. Typically, should it be impossible to sense any healthy natural heart rate in the atrium, for example in the case of atrial tachycardia or atrial fibrillation, cardiac pacemakers that are atrium-synchronous in principle often have a mode-switching capability in order to switch from an atrium-synchronous ventricular stimulation to an atrium-asynchronous stimulation in VVI mode, should a perceived atrial rate lie outside permissible limits. Generally, ventricular tachycardias within the scope of cardioversion therapy may be treated by stimulation with a stimulation rate above the tachycardia rate.
Typically, the stimulation modes may be set by corresponding control programs, which for example process or ignore detected events, or by control parameters. By way of example, generally, the detection of events in the atrium and/or ventricle may thus be activated or deactivated by a control parameter.
With regard to the references used herein, it is noted that the terms stimulation electrode or sensing electrode within the scope of the invention may include a respective electrode pole on an electrode line, for example the part of an electrode line via which stimulation pulses are delivered or electric potentials are received. It should also be noted that an electrode line used for stimulation may be referred to herein as a “stimulation electrode”.
Generally, the sensing electrode poles are connected during operation of the heart stimulator to corresponding sensing units, which may evaluate a respective electrocardiogram recorded via a sensing electrode pole (or a sensing electrode pole pair) and in particular may detect intrinsic atrial or ventricular events, that is to say natural atrial or ventricular contractions. Typically, this is achieved by way of example using a threshold value comparison, wherein an intrinsic event is detected when a respective intracardial electrocardiogram exceeds a threshold value predefined as suitable.
Generally, the respective intrinsic atrial heart rate (atrial frequency) or ventricular heart rate (ventricle frequency) may be derived from the frequency with which atrial or ventricular events follow one another, and for example tachycardias may thus be detected.
Typically, the detection of natural events is additionally used in demand pacemakers to suppress (inhibit) the delivery of stimulation pulses to a corresponding heart chamber should the natural event be detected in a time window prior to the planned delivery of a stimulation pulse to this heart chamber. In the case of rate-adaptive cardiac pacemakers, generally, the moment in time of the delivery of a respective stimulation pulse is planned in accordance with a respective stimulation rate, which is to correspond to the physiological demand of a patient, that is to say for example is higher with greater exertion. Generally, a heart stimulator may be equipped with one or more activity sensors, which for example may be a CLS (closed loop stimulation) sensor.
Typically, it is problematic that the function of such implantable electric medical devices, such as heart stimulators, may be severely adversely affected by strong electromagnetic fields or magnetic fields as occur for example in a magnetic resonance imaging (MRI) scanner (or magnetic resonance scanner or MRI scanner). Generally, many individuals who carry an active implantable medical device (also referred to hereinafter as an implant or IMD) are therefore contraindicated for MRI examinations, although MRI examinations are becoming increasingly important in the field of diagnostic medicine.
In order to still enable MRI examinations for individuals carrying active implantable medical devices, typically, various approaches are used, which are based either on the execution of the MRI examination or on the implantable medical device.
Generally, technologies for identifying magnetic fields are used, which are based on conventional methods for magnetic field detection. For example, United States Patent Application Publication US 2008/0154342, entitled “Implantable Medical Device Comprising Magnetic Field Detector”, to Digby et al., appears to describe a method for using a GMR (giant magnetic resistance) sensor in order to detect magnetic fields of MRI devices.
Generally, for example, an implantable cardioverter/defibrillator (ICD) is set by a cardiologist prior to an MRI examination into an operating mode that is not adversely affected by the magnetic fields prevailing in the MRI scanner. Following the MRI examination by a radiologist, typically, a cardiologist has to set the ICD back into an operating mode corresponding to the needs of the patient.